Pool boiling of R-134a/oil mixtures on enhanced tubes having different pore and gap sizes

The effect of enhanced geometry (pore diameter, gap width) is investigated on pool boiling of R-134a/POE mixture for enhanced tubes having pores with connecting gaps. Tubes having different pore diameters (and corresponding gap widths) are specially made. Significant heat transfer degradation by oil is observed for the present enhanced tubes. At 10% oil concentration, the degradation is 33–62% for Tsat = 4.4 °C. The degradation increases 58–77% at Tsat = 26.7 °C. The heat transfer degradation is significant even with small amount of oil (5–38% degradation at 1% oil concentration at Tsat = 4.4 °C), probably due to the accumulation of oil in sub-tunnels. The pore size (or gap width) has a significant effect on the heat transfer degradation. For most of the cases, the maximum degradation is observed for the largest pore/gap (0.27/0.10 mm) tube and the minimum degradation is observed for 0.23/0.10 mm tube. The highest heat transfer coefficient with oil is obtained for the largest pore/gap (0.27/0.10 mm) tube, which yielded the highest heat transfer coefficient for pure R-134a. The optimum tube significantly (more than 2 times) outperforms the smooth tube even with oil. The heat transfer degradation increases as heat flux increases.